The Federal Communications Commission and cable television testing organizations such as CableLabs have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG digital coding algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard in 1994 comprising for example, high definition television (HDTV) and standard definition (SDTV) digital signals for terrestrial broadcasting. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new ATV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
In 1994 the FCC will test the so-called "Grand Alliance" system, a system which is being cooperatively developed by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992. This system proposes to take the best features from those systems already tested in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has decided on a coding algorithm which will comply with the source coding standards proposed by MPEG (Motion Pictures Experts Group). In addition, an RF transmission approach developed by Grand Alliance member Zenith Electronics Corporation was selected by the Grand Alliance. Zenith utilizes a multi-level vestigial sideband (VSB) modulation approach which is described in "Digital Spectrum Compatible--Technical Details", Sep. 23, 1991 and in "VSB Transmission System: Technical Details", Dec. 17, 1993. Both of these publications are incorporated by reference herein.
The parent application from which the instant application depends, describes an improved approach to combatting co-channel interference which utilizes a generalized "rejection" filter at the receiver instead of the comb filter proposed by Zenith, and a generalized pre-coder at the transmitter. This approach solves a number of problems and improves performance when both co-channel NTSC and additive white Gaussian noise (AWGN) are present. The improved performance of the system is however, at the expense of some performance when only AWGN is present (e.g. 0.3 dB for a 36-tap filter). This loss in performance is due to the fact that the NTSC rejection filter is implemented at the decoder which causes noise enhancement because the noise also passes through the NTSC rejection filter.
A method for using a generalized pre-coder and a corresponding decoder for the case when uncoded QAM/VSB modulation is used, is described in "New Automatic Equalizer Employing Modulo Arithmetic", Electronic Letters, pp. 138-139, March 1971, by M. Tomlinson and in "Matched-Transmission Technique for Channels With Intersymbol Interference", IEEE Transactions on Communications, vol. COM-20, no. 4, pp. 774-780, August 1972, by H. Harashima and H. Miyakawa, which are incorporated by reference herein.
The implementation of a pre-coder along with a trellis encoder and the corresponding decoder is described in "Trellis Precoding: Combined Coding, Precoding and Shaping for Intersymbol Interference Channels", IEEE Transactions on Information Theory, vol. 38, no. 2, pp. 301-314, March 1992, by M. V. Eyuboglu and G. D. Forney, which is incorporated by reference herein. In this paper, it is shown that the asymptotic coding gain of the trellis code is unchanged when a pre-coder is used with the corresponding decoder for this case. The performance of a concatenated coding approach of using an outer RS (Reed-Solomon) encoder and an inner trellis encoder is dependent however, on the performance at larger error probabilities and not on asymptotic performance, as shown for example in "Practical Coding for QAM Transmission of HDTV" IEEE Journal on Selected Areas in Communications, vol 11, no. 1, pp. 111-118, January 1993, by C. Heegard, S. Lery and W. Paik, which is incorporated by reference herein.
Because the input to the Grand Alliance HDTV transmission system is an MPEG packet of 188 bytes, an RS code with parameters (208,188,10), which is a 10 byte error-correcting code, was chosen to be used with the Zenith approach. This choice is reasonable given the ease of implementation of implementing a RS decoder for byte-correction capability of up to 10-12 bytes.
TOV stands for the "threshold of visibility" of errors seen at the output of the MPEG decoder. Simulations have indicated that when a pre-coder designed to be used with a 36-tap rejection filter at the receiver, was used at the transmitter, the loss in TOV performance at RS packet-error rates of 6.times.10.sup.-4 was 0.9 dB using RS (208,108,10) and the trellis encoder proposed by Zenith. This significantly more than the predicted loss of 0.3 dB. In fact, in this case, even the asymptotic performance was degraded significantly.
In the original trellis-coding paper "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, vol. IT-28, no. 1, pp. 55-67, January 1982, by G. Ungerboeck, which is incorporated by reference herein, it was shown that the asymptotic performance of a trellis code is dependent either on the minimum distance between the signal points in a subset of the entire constellation (which is chosen in a particular way using a "set-partitioning" approach), or on the minimum distance path in the trellis corresponding to the convolutional code used within the trellis code. This trellis code dependent on the minimum distance of the subset of the entire constellation will be formed "subset-limited", while the trellis code dependent on the minimum distance path in the trellis will be termed "code-limited".
When the Zenith trellis code (which is "code-limited") is used, the pre-coder causes the trellis decoder to use subsets in an expanded constellation as described in "Trellis Precoding: Combined Coding, Precoding and Shaping for Intersymbol Interference Channels". The use of the expanded constellation changes the distance structure of the trellis corresponding to the convolutional code and thus causes a significant loss in performance for the case when a "code-limited" trellis code, for example, the Zenith trellis code, is used.
In the Zenith approach, a comb filter is used at the receiver to alleviate co-channel NTSC interference. At the transmitter, a 4-state trellis code with 8VSB modulation is used. It is then possible to treat the comb filter as a partial response channel in cascade with the 4-state trellis code and develop a Viterbi decoding strategy on an expanded trellis, the states of which correspond to the cascade of the states of the comb-filter and the trellis coder. This is explained in some detail in "VSB Transmission System: Technical Details". Clearly, these states are dependent on the comb filter.
If a switch is made at a packet boundary of the MPEG data stream to transmit, for example, a 4-state trellis code with 4VSB modulation instead of a 4-state 8VSB trellis code and the comb filter is not present, the same decoder is able to decode the 4-state 4VSB scheme, without causing any disruption at the boundary bits, if the same 4-state trellis code is used. If however, the comb filter is on, the switch between the expanded state 8VSB decoder to a four-state 4VSB decoder is not smooth. The memory introduced by the comb filter will cause degradation of performance at the boundary.
While it is possible to prevent this degradation by using a "dead" time in which symbols which are already known to the decoder are sent, this "dead" time must be greater than the memory of the comb filter, which has a memory of 12 symbols. This implies that after the transmission of every packet, at least 12 symbols with no particular purpose must be sent, which is an overhead of 1.4%.